One of the most basic, and important, measurements respecting electrical and electronic devices is the measurement of an unknown electrical current I. Current measurement has traditionally been performed by inserting a shunt of known resistance R in series with the current path, measuring the voltage drop V across the shunt, and converting the measured voltage to amperes (amps) by application of Ohm's Law: I=V/R.
To measure a wide range of currents, several different shunts are used. For example, a three-range ammeter (instrument that measures current) might use a shunt of 0.01 ohms for a measurement range of 0-10 amps (a voltage of 0.1 volt across such a shunt would indicate a current of 10 amps), a shunt of 0.1 ohms for a range of 0-1 amps (a voltage of 0.1 would indicate a current of 1.0 amps), and a shunt of 1.0 ohms for a range of 0-0.1 amps (a voltage of 0.1 would indicate a current of 0.1 amps).
A disadvantage of early ammeters was the need to manually substitute shunts, starting with the smallest resistance the operator believed to be appropriate for the current to be measured, until an appropriate reading could be obtained. Another disadvantage was that if the operator guessed wrong and inserted too high a shunt resistance, the resulting voltage would be too high and might make the voltmeter go off scale or even be damaged.
To overcome these disadvantages, ammeters with automatic range selection were developed. In such an ammeter, shunts would automatically be switched in and out one at a time, beginning with the smallest shunt resistance, until an appropriate measurement was obtained. Relays or switching transistors could be used for the switching. A dual-range electrical current measurement system is described in U.S. Pat. No. 6,603,301, issued to Michael Benes and assigned to the assignee of the present application. That disclosure represented a significant improvement over what was then the prior art as depicted in FIGS. 1 and 2 thereof. Referring to FIG. 4 thereof, a dual-range current measurement system includes a high-current sensor RH with an associated sense amplifier and a low-current sensor included within a 2-stage amplifier network. This network includes a bipolar current bypass consisting of FETs Q1 and Q2 that together shunt the current being measured to ground when the magnitude of that current exceeds the range of the low-current sensor.
Despite such advances in current measurement instruments, there remains a need for a way to measure an unknown electrical current with rapid settling time, glitch-free switching from one range to another, and minimum insertion impedance especially on low ranges.